NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Timekeeping and sight time records
From: Bill B
Date: 2005 Mar 18, 01:12 -0500
From: Bill B
Date: 2005 Mar 18, 01:12 -0500
> You are thinking about the difference between GPS (System) Time (which is not > updated for leap seconds) and UTC which is compensated for leap seconds to > keep it within 0.9 seconds of UT1. Currently the difference is 13 seconds. > Nothing to do with "satellite rollover." I get deeply confused on this subject at times. My Garmin 76 appears to display the same time as other online government sources (nanoseconds between glances at one or the other noted ;-) Somehow I thought the GPS difference was set back to zero when the satellites rolled over. Perhaps my recollection, or source (a GPS list years ago around the Y2K scare) is/was in error. I did read at: http://tycho.usno.navy.mil/gpstt.html "The number remains constant until the next leap second occurs. This offset is also given in the navigation (NAV) message and your receiver should apply the correction automatically. As of January 1, 1999, GPS time is ahead of UTC by thirteen (13) seconds." I enjoy the use of the word "should" when dealing in multiple seconds or nanoseconds.If I understand, the II/IIA satellite containing two cesium (Cs) and two rubidium (Rb) atomic clocks, and Block IIR satellite containing three Rb atomic clocks all report to a "paper" Composite Clock (CC). Then some black box factors in the missed leap seconds and viola, my 76 has the time to the nearest .9 seconds of some UTx standard. How does magic work? Bill